So far I have had good experience with kopia. But it is definitly less battle-tested than the other alternatives and I do not use it for too critical stuff yet.

As others said, Synapse can sometimes be very resource-hungry. It might be worth giving a try to Conduit and Dendrite, which are alternative Matrix server implementations and especially Conduit seems to be focused to by lightweight. Although I do not have any personal experience with them and it seems that they are most likely a lot less mature than Synapse at the moment.

Also see pairdrop, it is a snapdrop fork that allows connecting devices on different networks using a numeric code and has other improvements.